Hot-melt adhesives are adhesives which are solid at room temperature and which are applied to the substrates to be joined in the form of a melt, solidifying on cooling after the substrates have been joined together. In the case of thermoplastic adhesives, this operation can be repeated indefinitely because they do not cross-link. They are essentially based on polymers, such as polyamides, polyesters or polyolefins. These polymers typically determine the properties of the adhesive layer in regard to adhesion, strength and temperature behavior. In order to obtain special properties for specific applications, additives are incorporated, for example tackifiers to increase adhesion, plasticizers to increase flexibility, waxes to shorten the open time or crystallization accelerators to improve morphology.
Hot melt adhesives are used widely in the packaging industry for such applications as case and carton sealing, tray forming and box forming. The substrates to be bonded include virgin and recycled kraft, high and low density kraft, chipboard and various types of treated and coated kraft and chipboard. Composite materials are also used for packaging applications such as for the packaging of alcoholic beverages. These composite materials may include chipboard laminated to an aluminum foil which is further laminated to film materials such as polyethylene, Mylar, polypropylene, polyvinylidene chloride, ethylene vinyl acetate and various other types of films. Additionally, these film materials also may be bonded directly to chipboard or kraft. The aforementioned substrates by no means represent an exhaustive list, as a tremendous variety of substrates, especially composite materials, find utility in the packaging industry.
It would be advantageous to provide an adhesive composition that was a liquid at room temperature, a hot-melt solution at elevated temperatures, and a solid adhesive upon cooling.
For certain packaging and related applications, it may further be desirable to provide an adhesive that meets as many of the following technical specifications as possible: liquid at room temperature; stable over long periods (at least one year); chlorine free; low volatility; easily pumped by gravity feed to a gear or piston pump; largely unaffected by storage at 140 degrees F.°; low cost; ready to use as received (no mixing); will not “pack out,” separate, or change when pumped or pressurized; fuses to become a molten adhesive at elevated temperatures; FDA approved for indirect food contact; has good heat stability at fusion temperature; melt viscosity <10,000 cps; Tg similar to conventional EVA packaging grade hot melts; open molten time of about 7 seconds when applied at 350 degrees F.°; set time after compression between two substrates of about 2 seconds; capable of producing fiber tear adhesion to clay coated printed paperboard; foamable to a 50% density reduction while maintaining other adhesion specifications; and able to be compounded with up to 30 phr filler.